The sophistication of internal combustion engines and engine operating schemes continues to advance, and innovations in design and operation are revealed almost daily. One driving force behind many design changes in recent years has been increasingly stringent emissions requirements. One general approach to improving emissions quality relates to treatment of combustion products downstream from the engine. In other words, exhaust gases produced by the engine are treated via a variety of chemical and/or physical processes in an attempt to remove or reduce undesired constituents. In an alternative approach, engine developers have focused more on manipulating aspects of the combustion process itself to improve emissions quality.
Manipulation of fuel injection quantity, frequency, timing and even the type of injection spray pattern has been shown to facilitate reduction of certain undesirable emissions. Of particular interest to engineers are designs meeting or exceeding the government regulations relating to emission limits on various nitrogen-oxygen compounds, known collectively as “NOx”.
It has been discovered that enhancing mixing of air and fuel prior to ignition in an internal combustion engine cylinder can help reduce NOx levels in the engine exhaust. In the compression ignition, or diesel engine arts, one particular approach is known as “homogeneous charge compression ignition” or “HCCI”. In a homogeneous charge mode, fuel may be injected into an engine cylinder prior to the point at which the cylinder pressure and/or temperature is sufficient to trigger autoignition. This differs from a more traditional approach, wherein fuel is typically injected at or close to a point in the engine cycle at which autoignition can occur. In other words, in homogeneous charge operation, rather than fuel autoigniting upon injection, the fuel may be injected relatively earlier in the engine cycle, having more time to mix with air as the piston travels upward in the cylinder. As a result, ignition of most or all of the injected charge occurs at approximately the same time, rather than relatively gradually via a coherent flame front traversing the cylinder space.
While homogeneous charge operation has shown much promise in improving emissions quality in many vehicles, operation tends to be relatively sensitive to various operating conditions. Undesired variation in certain operating conditions, such as ignition timing, can compromise successful operation. Where a homogeneous charge engine is employed in an on-highway truck, for example, homogeneous charge failure can strand the truck far from a service station.
Various operating schemes and engine designs have developed over the years to reduce the likelihood of engine failures. In most engine designs capable of homogeneous charge operation, an electronic controller is coupled with several of the engine components, and controls various engine parameters to help reduce operating problems. However, even with the most advanced engine control systems, failures of course still occur.
U.S. patent application Ser. No. 10/372,692 to Shinogle et al., filed Feb. 21, 2003, now U.S. Pat. No. 6,997,159 discloses one operating design directed to providing an alternative operation scheme to allow an engine to operate under lower energy level conditions. While Shinogle appears to work well in its particular environment, there is always room for improvement and advancement of such strategies to other technical areas.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.